The term "quasar" originated as a contraction of "quasi-stellar radio source", because quasars were first identified as sources of radio-wave emission, and in photographic images at visible wavelengths they resembled point-like stars. High-resolution images of quasars, particularly from the Hubble Space Telescope, have demonstrated that quasars occur in the centers of galaxies, and that some quasar host galaxies are strongly interacting or merging galaxies.[3]

Quasars are found over a very broad range of distances (corresponding to redshifts ranging from z < 0.1 for the nearest quasars to z > 7 for the most distant known quasars), and quasar discovery surveys have demonstrated that quasar activity was more common in the distant past. The peak epoch of quasar activity in the Universe corresponds to redshifts around 2, or approximately 10 billion years ago.[4] As of 2017, the most distant known quasar is ULAS J1342+0928 at redshift z=7.54; light observed from this quasar was emitted when the Universe was only 690 million years old. The supermassive black hole in this quasar is the most distant black hole identified to date, and is estimated to have a mass that is 800 million times the mass of our Sun.[5][6][7]

Because quasars are distant objects, any light which reaches the Earth is redshifted due to the metric expansion of space.[8] Quasars inhabit the very center of active, young galaxies, and are among the most luminous, powerful, and energetic objects known in the universe, emitting up to a thousand times the energy output of the Milky Way, which contains 200–400 billion stars. This radiation is emitted across the electromagnetic spectrum, almost uniformly, from X-rays to the far-infrared with a peak in the ultraviolet-optical bands, with some quasars also being strong sources of radio emission and of gamma-rays.

Hubble images of quasar 3C 273. At right, a coronagraph is used to block the quasar's light, making it easier to detect the surrounding host galaxy.

Quasar QSO-160913+653228 is so distant its light has taken nine billion years to reach the telescope that took this photo, two thirds of the time that has elapsed since the Big Bang.[9]

In early optical images, quasars appeared as point sources, indistinguishable from stars, except for their peculiar spectra. With infrared telescopes and the Hubble Space Telescope, the "host galaxies" surrounding the quasars have been detected in some cases.[10] These galaxies are normally too dim to be seen against the glare of the quasar, except with special techniques. Most quasars, with the exception of 3C 273 whose average apparent magnitude is 12.9, cannot be seen with small telescopes.

Quasars are believed to be powered by accretion of material into supermassive black holes in the nuclei of distant galaxies, making these luminous versions of the general class of objects known as active galaxies. Since light cannot escape the black holes, the escaping energy is actually generated outside the event horizon by gravitational stresses and immense friction on the incoming material.[13] Central masses of 105 to 109solar masses have been measured in quasars by using reverberation mapping. Several dozen nearby large galaxies, with no sign of a quasar nucleus, have been shown to contain a similar central black hole in their nuclei, so it is thought that all large galaxies have one, but only a small fraction are active (with enough accretion to power radiation), and it is the activity of these black holes that are seen as quasars. The matter accreting onto the black hole is unlikely to fall directly in, but will have some angular momentum around the black hole that will cause the matter to collect into an accretion disc. Quasars may also be ignited or re-ignited when normal galaxies merge and the black hole is infused with a fresh source of matter. In fact, it has been suggested that a quasar could form when the Andromeda Galaxy collides with our own Milky Way galaxy in approximately 3–5 billion years.[13][14][15]

The Chandra X-ray image is of the quasar PKS 1127-145, a highly luminous source of X-rays and visible light about 10 billion light years from Earth. An enormous X-ray jet extends at least a million light years from the quasar. Image is 60 arcsec on a side. RA 11h 30m 7.10s Dec -14° 49' 27" in Crater. Observation date: May 28, 2000. Instrument: ACIS.

More than 200,000 quasars are known, most from the Sloan Digital Sky Survey. All observed quasar spectra have redshifts between 0.056 and 7.085. Applying Hubble's law to these redshifts, it can be shown that they are between 600 million[16] and 28.85 billion light-years away (in terms of comoving distance). Because of the great distances to the farthest quasars and the finite velocity of light, they and their surrounding space appear as they existed in the very early universe.

The power of quasars originates from supermassive black holes that are believed to exist at the core of all galaxies. The Doppler shifts of stars near the cores of galaxies indicate that they are rotating around tremendous masses with very steep gravity gradients, suggesting black holes.

Although quasars appear faint when viewed from Earth, they are visible from extreme distances, being the most luminous objects in the known universe. The brightest quasar in the sky is 3C 273 in the constellation of Virgo. It has an average apparent magnitude of 12.8 (bright enough to be seen through a medium-size amateur telescope), but it has an absolute magnitude of −26.7.[17] From a distance of about 33 light-years, this object would shine in the sky about as brightly as our sun. This quasar's luminosity is, therefore, about 4 trillion (4 × 1012) times that of the Sun, or about 100 times that of the total light of giant galaxies like the Milky Way.[17] This assumes the quasar is radiating energy in all directions, but the active galactic nucleus is believed to be radiating preferentially in the direction of its jet. In a universe containing hundreds of billions of galaxies, most of which had active nuclei billions of years ago but only seen today, it is statistically certain that thousands of energy jets should be pointed toward the Earth, some more directly than others. In many cases it is likely that the brighter the quasar, the more directly its jet is aimed at the Earth.

Quasars were much more common in the early universe than they are today. This discovery by Maarten Schmidt in 1967 was early strong evidence against the Steady State cosmology of Fred Hoyle, and in favor of the Big Bang cosmology. Quasars show the locations where massive black holes are growing rapidly (via accretion). These black holes grow in step with the mass of stars in their host galaxy in a way not understood at present. One idea is that jets, radiation and winds created by the quasars, shut down the formation of new stars in the host galaxy, a process called 'feedback'. The jets that produce strong radio emission in some quasars at the centers of clusters of galaxies are known to have enough power to prevent the hot gas in those clusters from cooling and falling onto the central galaxy.

Quasars' luminosities are variable, with time scales that range from months to hours. This means that quasars generate and emit their energy from a very small region, since each part of the quasar would have to be in contact with other parts on such a time scale as to allow the coordination of the luminosity variations. This would mean that a quasar varying on a time scale of a few weeks cannot be larger than a few light-weeks across. The emission of large amounts of power from a small region requires a power source far more efficient than the nuclear fusion that powers stars. The release of gravitational energy[18] by matter falling towards a massive black hole is the only process known that can produce such high power continuously. Stellar explosions – supernovas and gamma-ray bursts – can do likewise, but only for a few weeks. Black holes were considered too exotic by some astronomers in the 1960s. They also suggested that the redshifts arose from some other (unknown) process, so that the quasars were not really so distant as the Hubble law implied. This "redshift controversy" lasted for many years. Many lines of evidence (optical viewing of host galaxies, finding 'intervening' absorption lines, gravitational lensing) now demonstrate that the quasar redshifts are due to the Hubble expansion, and quasars are in fact as powerful as first thought.[19]

Animation shows the alignments between the spin axes of quasars and the large-scale structures that they inhabit.

Quasars have all the properties of other active galaxies such as Seyfert galaxies, but are more powerful: their radiation is partially 'nonthermal' (i.e., not due to black body radiation), and approximately 10 percent are observed to also have jets and lobes like those of radio galaxies that also carry significant (but poorly understood) amounts of energy in the form of particles moving at relativistic speeds. Extremely high energies might be explained by several mechanisms (see Fermi acceleration and Centrifugal mechanism of acceleration). Quasars can be detected over the entire observable electromagnetic spectrum including radio, infrared, visible light, ultraviolet, X-ray and even gamma rays. Most quasars are brightest in their rest-frame near-ultraviolet wavelength of 121.6 nmLyman-alpha emission line of hydrogen, but due to the tremendous redshifts of these sources, that peak luminosity has been observed as far to the red as 900.0 nm, in the near infrared. A minority of quasars show strong radio emission, which is generated by jets of matter moving close to the speed of light. When viewed downward, these appear as blazars and often have regions that seem to move away from the center faster than the speed of light (superluminal expansion). This is an optical illusion due to the properties of special relativity.

Quasar redshifts are measured from the strong spectral lines that dominate their visible and ultraviolet spectra. These lines are brighter than the continuous spectrum, so they are called 'emission' lines. They have widths of several percent of the speed of light. These widths are due to Doppler shifts caused by the high speeds of the gas emitting the lines. Fast motions strongly indicate a large mass. Emission lines of hydrogen (mainly of the Lyman series and Balmer series), helium, carbon, magnesium, iron and oxygen are the brightest lines. The atoms emitting these lines range from neutral to highly ionized, leaving it highly charged. This wide range of ionization shows that the gas is highly irradiated by the quasar, not merely hot, and not by stars, which cannot produce such a wide range of ionization.

This view, taken with infrared light, is a false-color image of a quasar-starburst tandem with the most luminous starburst ever seen in such a combination.

Since quasars exhibit properties common to all active galaxies, the emission from quasars can be readily compared to those of smaller active galaxies powered by smaller supermassive black holes. To create a luminosity of 1040watts (the typical brightness of a quasar), a super-massive black hole would have to consume the material equivalent of 10 stars per year. The brightest known quasars devour 1000 solar masses of material every year. The largest known is estimated to consume matter equivalent to 600 Earths per minute. Quasar luminosities can vary considerably over time, depending on their surroundings. Since it is difficult to fuel quasars for many billions of years, after a quasar finishes accreting the surrounding gas and dust, it becomes an ordinary galaxy.

Quasars also provide some clues as to the end of the Big Bang's reionization. The oldest known quasars (redshift ≥ 6) display a Gunn-Peterson trough and have absorption regions in front of them indicating that the intergalactic medium at that time was neutral gas. More recent quasars show no absorption region but rather their spectra contain a spiky area known as the Lyman-alpha forest; this indicates that the intergalactic medium has undergone reionization into plasma, and that neutral gas exists only in small clouds.

Like all (unobscured) active galaxies, quasars can be strong X-ray sources. Radio-loud quasars can also produce X-rays and gamma rays by inverse Compton scattering of lower-energy photons by the radio-emitting electrons in the jet.[22]

Picture shows a cosmic mirage known as the Einstein Cross. Four apparent images are actually from the same quasar.

The first quasars (3C 48 and 3C 273) were discovered in the late 1950s, as radio sources in all-sky radio surveys.[23][24][25][26] They were first noted as radio sources with no corresponding visible object. Using small telescopes and the Lovell Telescope as an interferometer, they were shown to have a very small angular size.[27] Hundreds of these objects were recorded by 1960 and published in the Third Cambridge Catalogue as astronomers scanned the skies for their optical counterparts. In 1963, a definite identification of the radio source 3C 48 with an optical object was published by Allan Sandage and Thomas A. Matthews. Astronomers had detected what appeared to be a faint blue star at the location of the radio source and obtained its spectrum. Containing many unknown broad emission lines, the anomalous spectrum defied interpretation — a claim by John Bolton of a large redshift was not generally accepted.

In 1962 a breakthrough was achieved. Another radio source, 3C 273, was predicted to undergo five occultations by the Moon. Measurements taken by Cyril Hazard and John Bolton during one of the occultations using the Parkes Radio Telescope allowed Maarten Schmidt to optically identify the object and obtain an optical spectrum using the 200-inch Hale Telescope on Mount Palomar. This spectrum revealed the same strange emission lines. Schmidt realized that these were actually spectral lines of hydrogen redshifted at the rate of 15.8 percent. This discovery showed that 3C 273 was receding at a rate of 47,000 km/s.[28] This discovery revolutionized quasar observation and allowed other astronomers to find redshifts from the emission lines from other radio sources. As predicted earlier by Bolton, 3C 48 was found to have a redshift of 37% of the speed of light.

So far, the clumsily long name 'quasi-stellar radio sources' is used to describe these objects. Because the nature of these objects is entirely unknown, it is hard to prepare a short, appropriate nomenclature for them so that their essential properties are obvious from their name. For convenience, the abbreviated form 'quasar' will be used throughout this paper.

Later it was found that not all quasars have strong radio emission; in fact only about 10% are "radio-loud". Hence the name 'QSO' (quasi-stellar object) is used (in addition to "quasar") to refer to these objects, including the 'radio-loud' and the 'radio-quiet' classes. The discovery of the quasar had large implications for the field of astronomy in the 1960s, including drawing physics and astronomy closer together.[30]

One great topic of debate during the 1960s was whether quasars were nearby objects or distant objects as implied by their redshift. It was suggested, for example, that the redshift of quasars was not due to the expansion of space but rather to light escaping a deep gravitational well. However a star of sufficient mass to form such a well would be unstable and in excess of the Hayashi limit.[31] Quasars also show forbidden spectral emission lines which were previously only seen in hot gaseous nebulae of low density, which would be too diffuse to both generate the observed power and fit within a deep gravitational well.[32] There were also serious concerns regarding the idea of cosmologically distant quasars. One strong argument against them was that they implied energies that were far in excess of known energy conversion processes, including nuclear fusion. At this time, there were some suggestions that quasars were made of some hitherto unknown form of stable antimatter and that this might account for their brightness.[citation needed] Others speculated that quasars were a white hole end of a wormhole.[33][34] However, when accretion disc energy-production mechanisms were successfully modeled in the 1970s, the argument that quasars were too luminous became moot and today the cosmological distance of quasars is accepted by almost all researchers.

In the 1980s, unified models were developed in which quasars were classified as a particular kind of active galaxy, and a consensus emerged that in many cases it is simply the viewing angle that distinguishes them from other classes, such as blazars and radio galaxies.[36] The huge luminosity of quasars results from the accretion discs of central supermassive black holes, which can convert on the order of 10% of the mass of an object into energy as compared to 0.7% for the p-p chainnuclear fusion process that dominates the energy production in Sun-like stars.

This mechanism also explains why quasars were more common in the early universe, as this energy production ends when the supermassive black hole consumes all of the gas and dust near it. This means that it is possible that most galaxies, including the Milky Way, have gone through an active stage, appearing as a quasar or some other class of active galaxy that depended on the black hole mass and the accretion rate, and are now quiescent because they lack a supply of matter to feed into their central black holes to generate radiation.

The energetic radiation of the quasar makes dark galaxies glow, helping astronomers to understand the obscure early stages of galaxy formation.[38]

Because quasars are extremely distant, bright, and small in apparent size, they are useful reference points in establishing a measurement grid on the sky.[39] The International Celestial Reference System (ICRS) is based on hundreds of extra-galactic radio sources, mostly quasars, distributed around the entire sky. Because they are so distant, they are apparently stationary to our current technology, yet their positions can be measured with the utmost accuracy by Very Long Baseline Interferometry (VLBI). The positions of most are known to 0.001 arcsecond or better, which is orders of magnitude more precise than the best optical measurements.

A multiple-image quasar is a quasar whose light undergoes gravitational lensing, resulting in double, triple or quadruple images of the same quasar. The first such gravitational lens to be discovered was the double-imaged quasar Q0957+561 (or Twin Quasar) in 1979.[40] A grouping of two or more quasars can result from a chance alignment, physical proximity, actual close physical interaction, or effects of gravity bending the light of a single quasar into two or more images.

As quasars are rare objects, the probability of three or more separate quasars being found near the same location is very low. The first true triple quasar was found in 2007 by observations at the W. M. Keck ObservatoryMauna Kea, Hawaii.[41]LBQS 1429-008 (or QQQ J1432−0106) was first observed in 1989 and was found to be a double quasar; itself a rare occurrence. When astronomers discovered the third member, they confirmed that the sources were separate and not the result of gravitational lensing. This triple quasar has a red shift of z = 2.076, which is equivalent to 10.5 billion light years.[42] The components are separated by an estimated 30–50 kpc, which is typical of interacting galaxies.[43] An example of a triple quasar that is formed by lensing is PG1115 +08.[44]

In 2013, the second true triplet quasars QQQ J1519+0627 was found with redshift z = 1.51 (approx 9 billion light years) by an international team of astronomers led by Farina of the University of Insubria, the whole system is well accommodated within 25′′ (i.e., 200 kpc in projected distance). The team accessed data from observations collected at the La Silla Observatory with the New Technology Telescope (NTT) of the European Southern Observatory (ESO) and at the Calar Alto Observatory with the 3.5m telescope of the Centro Astronómico Hispano Alemán (CAHA).[46][47]

When two quasars are so nearly in the same direction as seen from Earth that they appear to be a single quasar but may be separated by the use of telescopes, they are referred to as a "double quasar", such as the Twin Quasar.[49] These are two different quasars, and not the same quasar that is gravitationally lensed. This configuration is similar to the optical double star. Two quasars, a "quasar pair", may be closely related in time and space, and be gravitationally bound to one another. These may take the form of two quasars in the same galaxy cluster. This configuration is similar to two prominent stars in a star cluster. A "binary quasar", may be closely linked gravitationally and form a pair of interacting galaxies. This configuration is similar to that of a binary star system.